Since their introduction, there have been many popular and rare floppy disk types, densities, and formats used in computing, leading to much confusion over their differences.
TPI was not a part of the physical manufacturing process; it was a certification of how closely tracks of data could be spaced on the medium safely.
Subsequent use of the term "density" referred to physical characteristics of the media, with MFM assumed to be the logical format used.
When a soft-sectored disk is low-level "formatted", each track is written with a number of bytes calculated to fit within 360 degrees at the highest expected motor speed.
Special bit patterns are written right before the location where a sector should start, and serve as identifiers, similar to the punched holes used by hard-sectored disks.
If the motor is spinning any slower than the highest acceptable speed, which is usually the case, the data will fit in fewer than 360 degrees, resulting in a gap at the end of the track.
Commodore's Amiga used an unusual format which got closer to the disk's raw (unformatted) capacity by eliminating the gaps between sectors and simplifying the identification data.
[citation needed] For comparison purposes, formatted capacities given in this section assume standard disk geometries as they are supported by common operating systems in their default configuration.
SSDD originally referred to Single Sided, Double Density, a format of (usually 5+1⁄4-inch) floppy disks which could typically hold 35-40 tracks of nine 512-byte (or 18 256-byte) sectors each.